Powder lacquers based on epoxy-functional binding agents, such as, for example, glycidyl methacrylates (GMA), are known and are the subject of a large number of publications (see, for example, EP-B 0 509 393, page 2, lines 6 to 17).
Polycarboxylic acids, including in particular the aliphatic dicarboxylic acids, the anhydrides and polyanhydrides thereof, or even carboxy-functional polyesters or polyacrylates, are described as examples of suitable crosslinkers for powder-lacquer binding agents of such a type. Of the large number of possible crosslinker components that have been proposed, dodecanedioic acid or the polyanhydride thereof finds application almost exclusively in practice nowadays. Powder coatings cured with dodecanedioic acid are distinguished at comparatively moderate stoving temperatures by very good optical properties, in particular by outstanding flow, for which reason they have also been employed in the meantime as clear lacquer in the initial lacquering of automobiles.
However, with the exception of their good optics, the properties of powder lacquers based on epoxy-functional binding agents and dicarboxylic acids or dicarboxylic anhydrides as crosslinkers frequently do not satisfy the practical demands that are made of high-quality coatings. In particular, high resistances to chemical or mechanical loading, such as are known, for example, from conventional polyurethane lacquers cured with polyisocyanates, cannot be obtained with such powder-lacquer systems. This is due, on the one hand, to the strict difunctionality of the crosslinker components employed and the low crosslinking density associated therewith, and, on the other hand, to the fact that crosslinking of the system occurs exclusively via ester bonds which, in contrast to urethane bonds, for example, have distinctly higher sensitivity to hydrolysis or to the influence of chemicals.
Attempts have therefore also already been made to improve the properties of carboxy-functional and anhydride-functional crosslinkers by modification with polyisocyanates.
For instance, EP-A 0 695 771 describes conversion products of polyisocyanates with dicarboxylic (poly)anhydrides, which may optionally be present in a mixture with up to 50 wt. % of pure dicarboxylic acids, as crosslinker components for powder-lacquer binding agents based on epoxy resin. For the purpose of adjusting the molecular weight, low-molecular amines, alcohols or amino alcohols are optionally added in the course of synthesis of these crosslinkers. A very similar process for preparing polyisocyanate-modified polycarboxylic acids from dicarboxylic polyanhydrides is the subject of EP-A 0 874 010, wherein, however, water is added to the reaction mixture instead of amino-functional and/or hydroxy-functional molecular-weight regulators, in order to adjust the molecular weight of at least some of the anhydride structures selectively by hydrolysis. With both processes it is possible for branched crosslinkers to be obtained which, in combination with customary epoxy-functional powder-lacquer binding agents, yield higher-quality coatings. However, the synthesis of the polyanhydrides required as intermediate stage by conversion of dicarboxylic acids with acetic anhydride, subject to simultaneous removal of acetic acid by distillation, makes the preparation of these polyisocyanate-modified dicarboxylic (poly)anhydrides on a technical scale extraordinarily elaborate and expensive. In particular, by reason of their high cost the modified polycarboxylic acids according to EP-A 0 695 771 and EP-A 0 874 010 have been unable to gain market acceptance until now, despite their good crosslinker properties.
U.S. Pat. No. 5,371,167 also describes carboxy-functional polyisocyanate derivatives, prepared from polyisocyanates and compounds having at least one carboxyl group and at least one group, different from carboxyl, that is reactive with isocyanates, preferably hydroxycarboxylic acids or aminocarboxylic acids, which are employed in solvent-containing lacquer systems as crosslinkers for polyols, polyepoxides, polycarbodiimides or melamine-formaldehyde resins. Preparation of these carboxy-functional polyurethanes or polyureas is possible exclusively in solution under very mild conditions. For use as powder-lacquer crosslinkers, the products would therefore have to be freed from the solvent in an additional process step, which again is expensive. Preparation in a melt is not possible, by reason of the comparatively small differences in reaction between carboxyl groups and hydroxyl or amino groups in comparison with isocyanates.
The object of the present invention was therefore to make available a novel process for preparing polyisocyanate-modified carboxy-functional compounds, with which, starting from readily available raw materials, working can proceed easily, i.e. without elaborate process steps such as distillation for example, in a melt for example. The products obtained by this process have to enable, as powder-lacquer crosslinkers in combination with commercial powder-lacquer binding agents based on epoxy resin, in particular with glycidyl-functional polyacrylates, the preparation of high-quality powder coatings.